Geotechnical Engineering

DHARM

396 GEOTECHNICAL ENGINEERING

It = Influence Value, which is dependent on L/B

(This is analogous to Iσ of chapter 10).

For a perfectly flexible square footing, the immediate settlement under its centre is

twice that at its corners.

The values of It are tabulated below:

Table 11.2 Influence values for settlement of a corner of a flexible

rectangular foundation of size L × B (After Terzaghi, 1943)

L/B12345

Influence value It 0.56 0.76 0.88 0.96 1.00

As in the case of computation of the vertical stress beneath any point either inside or

outside a loaded area (chapter 10), the principle of superposition may be used for computing

settlement by using equation 11.4; the appropriate summation of the product of B and It for

the areas into which the total area is divided will be multiplied by q

F 1 −^2

HG

I

KJ

ν

Es

.

An earth embankment may be taken as flexible and the above formula may be used to
determine the immediate settlement of soil below such a construction.
Foundations are commonly more rigid than flexible and tend to cause a uniform settle-
ment which is nearly the same as the mean value of settlement under a flexible foundation.
The mean value of the settlement, Si, for a rectangular foundation on the surface of a semi-
elastic medium is given by:

Si = q. B

()

.

1 −ν^2

E

I

s

s ...(Eq. 11.5)

where B = width of the rectangular foundation of size L × B,

q = uniform intensity of pressure,

Es = modulus of elasticity of the soil beneath the foundation,

ν = Poisson’s ratio of the soil, and

Is = influence factor which depends upon L/B.

Skempton (1951) gives the following values of Is :

Table 11.3 Influence factors for mean value of settlement of a

rectangular foundation on a semi-elastic medium (After Skempton, 1951)

L/B Circle 1 2 5 10

Influence factor Is 0.73 0.82 1.00 1.22 1.26

The factor

F 1 −^2

HG

I

KJ

ν

Es

(Is) may be determined by conducting three or more plate load tests

(Chapter 14) and fitting a straight line plot for Si versus q. B; the slope of the plot equals this
factor.